Clean up menu structure on wiki

Two problems several people have identified:

• The current top menu on the wiki is not clear or useful (for instance, not clear that "home" should go to the main mastodon server, since the wiki's main page is also called "Home.")
• The top menu takes up a lot of vertical real estate

Proposed solutions:

• Change menu items to:
  • Social.coop (links to https://social.coop/about/more)
  • Governance (links to /Governance
  • Operations (links to /Operations)
  • Platforms (links to /Platforms)
  • Join (links to /registration-form)
• Reduce the space between the page title and the content
• Make Social.coop logo smaller and have it link to / rather than https://social.coop